Molecular systematics of North American Eupatorium

by Siripun, Kunsiri Chaw.

Abstract (Summary)

This study had two primary goals: first, to prepare a thorough taxonomic revision
of North American Eupatorium, and second, to conduct initial evaluations using
molecular techniques of how diploid and polyploid populations of Eupatorium
rotundifolium and E. sessilifolium relate to one another. Nuclear ribosomal internal
transcribed spacer (ITS) region and chloroplast DNA (trnC-psbM) sequences were used
for molecular analysis, which allowed assessment of relationships of the polyploid
samples based on both a biparentally and a maternally inherited marker. The Inter
Simple Sequence Repeat (ISSR) method was used for examining the relationships among
diploid and polyploid populations including the genetic variation between the species.
The number and variation in band-patterns between diploid and polyploid populations
was examined to answer the question of whether natural hybridization was involved. The
trees that were generated from the ISSR data gave evidence on the levels of genetic
variation within and between populations of both species, as well as allowing evaluation
of whether polyploidy has arisen from autopolyploidy or allopolyploidy.
A total of 20 species of Eupatorium occur in North America. The primary species
concept that was employed for classification of Eupatorium in North America was a
morphological one. Species were recognized that could be consistently distinguished by
features of the capitulescence, numbers of florets per head, shape and arrangement of the
involucral bracts, and features of the leaves including size, shape, margin, and venation.
This study provides an example of the successful use of ITS, chloroplast DNA
sequence data, and ISSR methods to reconstruct reticulate evolution in plants and
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confirms that molecular data can be highly informative, even when there is a limitation of
morphological characters. The result provided the evidence to suggest that polyploid
populations within both Eupatorium rotundifolium and E. sessilifolium are
allopolyploids. The evidence of molecular approaches was consistent with the
explanation that these two species showed reticulate evolution. Polyploid populations
may have arisen many times from multiple origins, and this was particularly clear in the
polyploid, apomictic species Eupatorium godfreyanum. Because ITS and chloroplast
DNA sequence data provided low levels of variation at the intraspecific level, the
analysis was supplemented with data from the ISSR methods to investigate in detail the
relationships among populations within each species and to assess more thoroughly the
origin of polyploidy in each.
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